My question is on the <dc:identifier>. Is the <dc:indentifier> unique and hashed to an IP address or MAC address?

2 Answers 2


The dc:identifier is commonly an ISBN, though any sort of URN will do. Further information about using in epub can be found on the IDPF's site, but as a quick example, you could use something like:

<dc:identifier id="isbn">urn:isbn:0451450523</dc:identifier>

As long as the identifier is unique, it should be fine. At the above link, the IDPF says:

Every metadata section must include at least one identifier element containing an unambiguous identifier for the Publication. Multiple identifier elements are permitted, but only one can be marked as the Unique Identifier via the package element unique-identifier attribute.

  • 3
    Because of the cost involved, only those EPUB 3.0 publications that are commercially published can be expected to use an ISBN as primary ID. So although these can, and will exist, they are not common, and will be a tiny minority of all EPUB 3.0 publications. I have a 5000+ EPUB 3.0 files and none of them have an ISBN and all have a UUID as primary ID.
    – Anthon
    Mar 4, 2014 at 18:59
  • 1
    @Anthon, bear in mind that cost depends to a certain extent on where you are. For example, ISBNs are free in Canada, apparently.
    – Tom
    Jun 2, 2014 at 18:31

Yes it should be unique, and no, it is not hased to an IP or MAC address.

The example in the EPUB 3.0 specification uses a UUID. The particular UUID used in the example is:


And the 4 in the position directly after the second - indicates that it is a random UUID. The UUID linked to a MAC address is version 1.

There is however, no necessity that this is a UUID. The only premise is that

a primary identifier that is unique to one and only one particular EPUB Publication

E.g. the epub 3 examples mostly use a URN, and AFAIK none of those use an ISBN.

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